Automatic frequency control for pulse transmission systems



May l0, 1,949.

G; w. FYLER 2,469,875 AUTOMATIC FREQUENCY CONTROL FOR PULSE TRANSMISSIONSYSTEMS Filed JNM-26, 1944 is Attorney.

Patented May 1 0, 1.949

UNITED STATES PATENT vori-ica l AUTOMATIC FREQUENCY CONTROL FOR l PULSETRANSMISSION SYSTEMS' George W. Fyler, Stratford, Conn;, assignor toGeneral Electric New York Company, a corporation of Application January26, 1944, Serial No. 519,709

, transmitted pulses.

A further object of my invention is to eiect such regulation in responseto waves received only while the transmitter is operating, i. e. duringthe vtransmitted pulses.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims. My inventionitself, however, both as to its organization and method of operation,together with further objects and advantages thereof, may best beunderstood by reference to the following description taken in connectionwith the' accompanying drawing in which Fig. l represents an embodimentof myinvention; Figs. 2 and 3 represent certain characteristicspertaining to the oscillator employed therein; and Fig. 4 representscharacteristics of the frequency discriminator employed therein.

Referring to Fig. 1 of the drawing, I have indicated in the upperleft-hand corner an antenna I which may be supplied with from atransmitter 2 vto be radiated into space and thereafter to be receivedin a receiver comprising the usual so-called transmit-receivev device orTR box 3, mixer 4, IF amplifier 5, detector 5 and echo range indicatorl. The carrier wave transmitted may be one of short wavelength, as forexample ten centimeters, and may be emitted by the antenna I as 'pulsesof one to ten microseconds duration, for example. While the antenna maybe one of any desired type, it is indicated as arranged within aparabolic reflector 8 arranged for variable orientation by mountingmeans 9.

' The TR box may comprise the usual equipment adapted to protect theremaining portion Aof the receiver from the intense pulses producedI bythe transmitter, while permitting the transmission of received pulsesreected from remote objects to the mixer 4. For further information onvarious devices suitable for this purpose, reference may be made to"Principles of Radar, 2nd ed.,

carrier wave pulses 5 Claims. (Cl. 343-13) 2 1946 (McGraw-Hill Book C0.,Inc.), chapter XI, entitled Transmit-receive devices,

The mixer 4 comprises means for mixing' the received Vcarrier wavepulses with an additional carrier wave which may be of either higher orlower frequency but which is generated by an oscillator I0 and suppliedto the mixer 4 over conductors I3. These oscillations are mixed in themixer 4 to produce oscillations of beat frequency corresponding to thedifference in frequency between the two carrier waves. These beatfrequency oscillations are amplied by the intermediate frequencyamplifier 5, rectified by the detector 6 and supplied to the echo rangeindicator l.

The local oscillator II)l comprises a cavity resonator I I mounted aboutthe evacuated envelope I4 of an electron discharge device. This electrondischarge device comprises a cathode I5 and an anode, or repeller, I8spaced apart and having interposed therebetween two electron permeablewalls or grids I 'I and I8, these walls being positioned in alignmentwith and electrically connected to the respective upper and lower wallsI9 and 20 of the cavity resonator and forming in effect electricalcontinuations thereof within the envelope of the evacuated vessel I4.Thus both the upper and the lower walls of the resonator I I areelectrically continuous from the outer periphery 23 of the resonator tothe central axis thereof along which the beam of electrons from thecathode to the anode passes, being broken only by the thickness of theglass wall of the evacuated vessel I4.

The resonator II is connected to the positive terminal of a source ofoperating potential the opposite conductors of which are indicated at24. This source'of voltage, for example, may be of the order of 500vo1ts, its lpositive terminal being at fromj200 to 300 volts (e. g., inthe neighborhood of 250 volts) positive with respect to ground and itsnegative terminal being at a desired voltage below ground. This sourceis shunted by a bleeder resistance 25 the intermediate point of which isgrounded. This intermediate point may be at any suitable point on theresistance and not necessarily at the midpoint. 'I'he negative terminalof the source is connected through a variable resistance 26 to thecathode of an electron discharge device 21 the anode of which isconnected to the anode I6 of the local oscillator I0. This dischargedevice 2l is connected to regulate the potential between the cathode I5and repeller I6, thereby to regulate the frequency of the localoscillator to maintain the beat or in- 'f stated briefly.

termediate frequency supplied to amplier suby stantially constant.

rectifier 30, which is either positive or negative with respect to thenegative terminal of the source 24 dependent upon the frequency o f theoscillations supplied to the discriminator 29 and which is of magnitudedependent upon the variation in that frequency from the frequency towhich the circuits of the discriminator 29 are resonant.

The operation of this discriminator circuit is well known to thoseskilled in the art but will be The primary and secondary circuits of thetransformer 34 are both resonant at the intermediate frequency to bemaintained. Oscillations in the secondary circuit 35 are therefore inquadrature relation with the oscillations which appear across theprimary winding of the transformer 34. The oscillations on the primarywinding are supplied through condenser 31 to 'the anodes of the twodiodes 30 and 33 in phase through resistances 38 andf39. Theoscillations in the secondary circuit 35 are supplied to these anodes inopposed phase relation. Thus if the two oscillations on' the two anodesyare in exactly quadrature relation, equalk voltages exist on the twoanodes and equal and opposite unidirectional voltages are thereforeproduced on the two resistances 43 and 44 with the result that zerovoltage is produced between the two cathodes of the rectiers 3l) and33.' If the frequency supplied to the discriminator changes from that towhich the cirf cuits of the transformer 34 are resonant, then the twooscillations on one of the two anodes become more nearly in phase whilethose on the other anode become more nearly in opposed phase with theresult that the potential increases on one of the resistances 43 and 44and decreases on the other. Of course, the reverse occurs if thefrequency of the oscillations varies in the opposite direction. Thus avoltage appears at the cathode of rectifier 30 which is either positiveor negative with respect to the negative terminal of the source 24dependent pon the direction of the frequency variation and of magnitudedependent upon the extent of the variation. This potential is suppliedto the control electrode of the discharge device 21 to control theconductivity thereof.

The cathode of the device 21 is connected to ground, which is morepositive than the cathode, through two resistances 45 and 46 the anodethereof being connected through a load resistance 41 to a variable pointon the resistance 46 and the screen grid 48 being connected to avariable point on the resistance 45. The suppressor grid 49 is connecteddirectly to the cathode. Of course, current owing in device 21 flowsthrough resistance 41 and the potential on this resistance governs thepotential on the repeller I6 with respect to the cathode I5 ofoscillator I0.

The amplifier 28 is of the usual pentode type having its suppressor gridconnected to ground and its screen grid connected to the cathode of afurther electron discharge device 5|), this cathode being connected toground through a resistance 53. This latter discharge device 50 isnormally nonconducting so that the potential across resistance 53 iszero and therefor the amplifier 28 is rendered inoperative by the zeropotential on its screen grid. Device 50, however, is rendered,conducting by a pulse supplied from the transmitter 2 over conductor 54during each radiated pulse. Current then flows in the resistance 53during each radiated pulse rendering its cathode and hence the screengrid of device 28 positive. Thus this device 28 is then capable ofsupplying oscillations of intermediate frequency from the intermediatefrequency amplifier to the discriminator to control the conductivity ofthe de vice. 21 thereby to control the potential on the anode I6.

While as previously stated the TR box operates to protect the mixer 4from the intense pulses produced by the transmitter, oscillationsproduced by the transmitter nevertheless are transmitted through the TRbox to the mixer 4 during the radiation of pulses to an extentsuillcient to produce a beat frequency which is supplied to theintermediate frequency amplifier and thence through amplifier 28 to thediscriminator 29 to control the conductivity of device 21 thereby tocontrol the potential on the anode or repeller I6. Since amplifier 28 isoperative only during the radiated pulses, the control of the localoscillator rfrequency is not subject to noise Voltage or otherextraneous electromotlve forces received during the intervals betweenthe radiated pulses.

Condenser 55, which is connected in shunt with resistances 43 and 44, isof such a value that the unidirectional potential produced between thetwo cathodes of rectiers 30 and 33 is maintained during the periodbetween transmitted ypulses and varies substantially only as the beatfrequency varies during the radiated pulses. W

The discharge device I4 is commercially available and is known by thecommercial designation 101B. This device operates by the passage ofelectrons from the cathode to the anode I6 through the two electronpermeable walls or grids I1 and I8. If it be assumed that oscillationsare excited in the resonator II due to variations in the electronstream, it will be understood that electrons which are in the spacebetween walls I1 and I8 when wall I1 is positive with respect to wall I8are accelerated. Similarly electrons in this space when wall I1 isnegative with respect to wall I8 are decelerated. The distance betweenthe upper and lower walls I1 and I8 is such that an electron traversesthis space in a half cycle 0r less. The result is that electrons leavingthe wall I1 and traveling toward the anode I6 are in alternate groups of'fast and slow electrons. The fast electrons, due to their. velocity,may impinge upon the anode I6 and produce current in the anode circuit.The slow electrons are repelled by the negative voltage of the anode 'I6and travel back toward the wall I1 and enter the space within theresonator. If the distance which they travel from the wall I1 toward theanode I6 and then back to the wall' I1 is such that the groups of slowelectrons arrive back in phase with the oscillations in the resonator,they operate to enhance or sustain the -oscillations within theresonator. As this distance varies, however, as would be the case if thepotential on the repeller I6 varied, then the electrons arrive back atthe wall I1 in phase to produce a shifting of the frequency of theoscillations withinthe device. In this way, by varying the voltage onthe repeller, the frequency of the oscillations within the resonator maybe Varied over a range of frequencies.

Of course, the variation in potential on the repeller I6 not only variesthe distance traveled by the slow electrons toward the repeller, but

it also varies the velocity of the electrons passing from 4wall I1toward the repeller, and these two effects may be counteracting, Inpractice,

however, it is found that ifthe repeller voltage is increased in anegative sense with respect to the cathode I5, the frequency isincreased.

A plurality of plungers are arranged in the cavity on screw-threadedstems which project through the wall thereof and which are provided withnuts on the outside adjustable to vary the position of the plungersthereby to vary the frequency of resonance of the cavity.

Fig. 2 represents certain characteristics of this oscillator. Thisfigure shows by curves A and B the relation between the intensity ofoscillations produced plotted as ordinates and the negative repellervoltage plotted as abscissa. While the device may have several modes ofoperation, only two are indicated by Fig. 1, one at a repeller voltagerepresented by the point 63 and another at a repeller voltagerepresented by the point 64,

on the abscissa, the oscillations of each mode having maximum intensitywhen the repeller voltage is of the value corresponding to therespective points 63 and 64 and reducing in intensity as the repellervoltage either increases or decreases. If the negative repeller voltageincreases, however, the frequency of the oscillations increases, whereasif the negative repeller voltage decreases, the frequency decreases.This relationship is illustrated in Fig. 3 by curves C and D. The shapeof these curves varies with` the repeller voltage and may be nearlylinear. Thus by proper regulation of the repeller voltage, the frequencyof the local oscillator may be varied to maintain the intermediatefrequency constant, notwithstanding variations in frequency of thecarrier wave produced by the transmitter 2.

It is desirable, however, that the intensity of these oscillationsshouldnot become too small for satisfactory operation of the converteror mixer 4, and satisfactory productions of oscillations ofinterdmediate frequency, and that it should not vary beyond valuesrepresented by lines 66 and 68. Such variation is prevented by thelimiting action of device 21 presently to be described.

In Fig. 4, the curve E represents the relation between unidirectionalvoltage produced between the cathodes of diodes 30 and 33 plotted asordi-- nates and frequency of oscillations supplied through amplifier 28plotted as abscissa, the xed frequency to which the circuits of thetransformer 34 are tuned being represented by the point 65. When thefrequency is at the correct value, the unidirectional voltage betweenthe two cathodes of the two diodes 30 and 33 is zero. As the frequencydecreases from this value, this voltage increases with one polarity, andas the frequency increases the voltage increases with the oppositepolarity. If the voltage increases in the positive sense, it causesdischarge device 21 to become increasingly conductive until a value isreached, represented by the line 66 in Fig. 3, at which the controlelectrode of discharge device 21 becomes positive with respect to thecathode and therefore draws current through resistance 61.` Thisresistance is sufficiently great so that the control electrode cannotbecome more positive. If the unidirectional potential between thecathodes of the diodes 3l! and 33 varies in the opposite direction to avalue represented by the line 63', then the cutoi point of the device 21is reached and it becomes nonconductive so that no further change in therepeller voltage occurs. The adjustment ofthe variable contacts onresistances 45, 46 and 26 may be such that these two lines 66 and 68correspond to vertical lines 66 and 68 drawn in the curve B of Fig. 2,showvents the oscillations of the local oscillator from becoming of toolow intensity for satisfactory reception due to variation in therepeller voltage through the control action of the regulator.

Of course, due to the amplifying action of device 21 the repellervoltage changes much more rapidly with the change in frequency than doesthe unidirectional voltage between cathodes of diodes 30 and 33.

The adjustment of the variable contact on resistance 46 determines theanode operating voltage of device 21 and hence selects the load line onwhich device 28 operates. That is, if the anode-current anode-voltagecharacteristics of device 28 be drawn for different values of gridvoltage, a so-called load line may be drawn across these characteristicsfor each value of anode voltage. A particular load line is selected bythe position of the variable contact on resistance 46. This adjustmentmay be such that the repeller voltage of device I0 varies about thepoint 63 or 6`4 of Fig. 2. The range of repeller voltage variation maybe adjusted by varying the position of the contact on resistance 45thereby varying the'screen grid voltage of device 21. The cathoderesistance 26 determines the bias on device 26 and the point onl theparticular load line about which the grid voltage is varied by variationin voltage between the cathodes of diodes 30 and 33.

While I have shown a particular embodiment of my invention, it will ofcourse be understood that I do not wish to be limited thereto asdifferent modifications both 'as 'to the circuit arrangement and theinstrumentalities employed may be made, and AI therefore contemplate bythe appended claims to cover any such modifications as fall within thetrue spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. In combination, in a carrier wave pulse system, a carrier wave pulsetransmitter, a carrier Wave pulse receiver having a local oscillator,said i receiver being arranged to receive the carrier wave pulsestransmitted by said transmitter and to combine them with oscillationsproduced by said local oscillator to produce a beat frequency, meansresponsive to said beat frequency to control the frequency ofoscillations produced by said local oscillator to maintain said beatfrequency constant irrespective of variation in frequency of thelcarrier Wave of the transmitted pulses, and means means normallyinoperaoperative only during said means controlled by said discriminatorto regulate the frequency of said local oscillator to maintain said beatfrequency constant. and means to render operative said normallyinoperative means during the period of each transmitted pulse.

3. In combination, a carrier wave pulse transmitter, a receiver arrangedto receive carrier wave pulses from said transmitter, said receiverhaving a local oscillator comprising an anode, a cathode, and a cavityresonator having walls interposed between said anode and cathode throughwhich electrons pass to excite oscillations in said Acavity resonator,means to combine said oscillations with oscillations received from saidtransmitter to produce oscillations of a beat frequency, meanscontrolled by said beat frequency to regulate the potential between saidanode and cathode to maintain said beat frequency constant, and means tomaintain said last means inoperative and to render it operative duringpulses transmitted by said transmitter.

4. In combination, means to transmit oscillations in recurrent pulses,means to receive said oscillations both directly and after reflectionfrom a remote body, a local oscillator heterodyning with said receivedoscillations to produce a beat frequency, means to produce aunidirectional potential of value dependent upon said beat frequencywhen said oscillations are received directly, means to maintain saidunidirectional potential throughout reception of said oscillations afterreflection, and means to regulate the frequencyof said local oscillatorin accord with said unidirectional potential.

5. In combination, means to transmit oscillations in recurrent pulses,means to receive said oscillations both directly and after reflectionfrom a remote body, an oscillator heterodyning with said receivedoscillations to produce a beat frequency, said oscillator comprising anelectron discharge device having an anode and a cathode and beingadapted to produce oscillations of frequency dependent upon thepotential between said anode and cathode, means to produce aunidirectional potential between said anode and cathode dependent uponthe frequency `of said beat note produced only during reception of saiddirectly received oscillations, and means to maintain said potentialthroughout the interval ,between said transmitted pulsesand duringreception of said oscillations after reection.

GEORGE W. FYLER.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATESV PATENTS Number Name Date Re. 21,691 Freeman Jan. 14, 19412,189,549 Hershberger Feb. 6, 1940 2,231,368 Mountjoy Feb. 11, 19412,233,165 Goldman Feb. 25, 1941 2,268,643 Crosby Jan. 6, 1942 2,421,020Earp May 27, 1947 FOREIGN PATENTS Number Country Date 113,233 AustraliaJune 2, 1941 Disclaimer 2,4-@4 8^75.-Geo7"ge W. F11/lef', Stratford,Conn. AUTOMATIC FREQUENCY CON- TROL FOR PULSE TRANSMISSION SYSTEMS.Patent dated May 10, 1949.

Disclaimer filed Jan. 3l, 1952, by the assignee, Gene/MZ Elec/fm2: 00m'-pany. Hereby enters this disclaimer to Claims l v 2, 4, and 5 of saidpatent. [Ocz'al Gazette March 4, 1952.]

MEF-George W. Fg/Ze'", Stratford, Conn. AUTOMATIC FREQUENCY CON- TROLFOR PULSE TRANSMISSION SYSTEMS. Patent dated May 10, 1949.

Disclaimer filed Jan. 81, 1952, by the assignee, General Elecm'c00mpany. Hereby enters this disclaimer to Claims 1, 2, 4, and 5 of Saidpatent.

[Oficial Gazette March ,4, 1.952.]

